This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-330657 filed in Japan on Nov. 15, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to printed circuit boards and methods for manufacturing the same in which a circuit pattern is insulated by a film cover layer.
Flexible printed circuit boards and the like (hereinafter simply referred to as “printed circuit board”) are known as conventional printed circuit boards provided with a film cover layer.
A conventional printed circuit board is described using FIGS. 9 to 14.
FIG. 9 is a cross-sectional view showing a schematic cross section of a printed circuit board according to Conventional Example 1.
FIG. 10A is an explanatory diagram illustrating a schematic broken plane of principal components of the printed circuit board shown in FIG. 9 using an isometric projection technique and FIG. 10B shows an enlargement of an opening indicated by a circle portion B in FIG. 10A. It should be noted that in FIG. 10A, a plating layer is shown cut midway in the opening, and in FIG. 10B the plating layer is omitted and a connecting portion is exposed.
A base material of a printed circuit board 101 of Conventional Example 1 is constituted by an insulating layer 110 and a conductive layer 111 laminated on the insulating layer 110. The conductive layer 111 is subjected to appropriate patterning, thereby forming a connecting portion 112 and a circuit pattern portion 113 (process of forming a wiring pattern). The connecting portion 112 includes terminal portions for achieving connection externally, and the circuit pattern portion 113 is a circuit wiring portion that is integrally formed connected to the connecting portion 112.
The connecting portion 112 and the circuit pattern portion 113 are covered by a film cover layer 115 via an adhesive layer 114 constituted by a thermosetting adhesive agent to secure mutual insulation between the outside and the wiring pattern and to improve reliability (process of forming a cover layer).
It should be noted that after an opening 116 is opened in advance so as to correspond to the connecting portion 112, the adhesive layer 114 and the film cover layer 115 are aligned with the insulating layer 110 and the conductive layer 111 so as to abut thereagainst, and thermocompression bonded by being subjected to heat and pressure.
After the film cover layer 115 has been placed, a plating layer 117 is formed on (a top surface 112s of) the connecting portion 112 so that a mounted component 120, such as an electrical component to be mounted, can be connected to the connecting portion 112.
When the film cover layer 115 is thermocompression bonded to the base material (the insulating layer 110 and the conductive layer 111), the adhesive layer 114 outflows in a horizontal direction since it is compressed by the applied pressure such that it is pushed out to form an adhesive agent outflow portion 114a. That is, adhesive agent protrudes from the adhesive layer 114 at the opening 116 that is provided in advance in the film cover layer 115.
Since the adhesive agent outflow portion 114a covers the top surface 112s of the connecting portion 112, a region formed by the plating layer 117 becomes covered by the adhesive agent such that sometimes a sufficient connecting surface area cannot be achieved between the plating layer 117 and the connecting portion 112.
When there is not a sufficient connecting surface area between the plating layer 117 and the connecting portion 112, there are problems such as reduced reliability due to increased contact resistance with the mounted component 120 and at times also connection defects, which result in reduced yields.
The following techniques (Conventional Example 2 to Conventional Example 5) have been conceived as measures to address this problem.
In Conventional Example 2, in order to suppress the adhesive agent outflow portion 114a that flows out to be formed at the opening 116 of the film cover layer 115, an adhesive layer 114 (a film cover layer 115) is employed in which the adhesive agent has little outflow volume.
However, even when selecting and using an adhesive layer in which the adhesive agent has little outflow volume and the adhesive agent outflow portion 114a that has been formed is small, unevenness is produced in the adhesive agent outflow portion 114a due to unevenness in the thickness and hardness of the film cover layer 115 and/or the adhesive layer 114 in each lot.
In particular, since a physical means for suppressing outflow of the adhesive agent outflow portion 114a is not provided in the connecting portion 112, which requires the adhesive agent outflow portion 114a to be suppressed, there is the problem that it is difficult to suppress the formation of the adhesive agent outflow portion 114a to the connecting portion 112 and that the plating layer 117 cannot be formed stably.
Conventional example 3 involves inhibiting and controlling the pressing force onto the film cover layer 115 by a thermal press when thermocompression bonding is carried out.
This has the problems that configuring (the thickness and hardness) of a cushioning material arranged between the thermal press and the printed circuit board 101 (film cover layer 115) in thermocompression bonding and managing the pressure in thermocompression bonding are difficult, that there is unevenness in the thickness and hardness of the adhesive layer 114 depending on the lot of the film cover layer 115, and moreover that there are differences in the outflow volume of the adhesive agent outflow portion 114a due to which position the opening is provided is in the printed circuit board.
In particular, because thermocompression bonding is carried out on a flat board shape, the pressure load varies depending on whether or not there is the conductive layer 111 (the connecting portion 112 and the circuit pattern portion 113), thereby producing differences in the shape of the adhesive agent outflow portion 114a. Furthermore, as in Conventional Example 2, since a physical means for suppressing outflow of the adhesive agent outflow portion 114a is not provided in the connecting portion 112, there is the problem that it is difficult to suppress the formation of the adhesive agent outflow portion 114a to the connecting portion 112 and that the plating layer 117 cannot be formed stably.
FIG. 11 is a cross-sectional view showing a schematic cross section of a printed circuit board according to Conventional Example 4. FIG. 12A is an explanatory diagram illustrating a schematic broken plane of principal components of the printed circuit board shown in FIG. 11 using an isometric projection technique and FIG. 12B shows an enlargement of an opening indicated by a circle portion B in FIG. 12A.
It should be noted that in FIG. 12A, a plating layer is shown cut midway in the opening, and in FIG. 12B the plating layer and an ink cover layer are omitted and a connecting portion is exposed. Furthermore, identical symbols are attached to the same structures as in Conventional Example 1 and detailed description thereof is omitted.
In Conventional Example 4, an ink cover layer 115a is printed to achieve insulation in locations where control management of (the outflow volume of adhesive agent of) the adhesive agent outflow portion 114a is difficult.
Such a technique is generally used in the case of intricate wiring patterns and involves making in advance the opening 116 of the film cover layer 115 larger than required, then performing thermocompression bonding on the film cover layer 115, after which the ink cover layer 115a is printed to form an intricate insulating layer (the ink cover layer 115a) and an opening 115b. 
Since Conventional Example 4 involves providing in advance the opening 116 larger than required in the film cover layer 115 then printing the ink cover layer 115a after performing thermocompression bonding on the film cover layer 115, it is possible to ensure that there is no adhesive agent outflow portion 114a produced on the connecting portion 112 (top surface 112s). However, costs are increased since there is an increase in the materials used and in the processing; moreover there is a risk of reduced yields when printing defects occur during printing of the ink cover layer 115a. 
FIG. 13 is a cross-sectional view showing a schematic cross section of a printed circuit board according to Conventional Example 5. FIG. 14A is an explanatory diagram illustrating a schematic broken plane of principal components of the printed circuit board shown in FIG. 13 using an isometric projection technique and FIG. 14B shows an enlargement of an opening indicated by a circle portion B in FIG. 14A.
It should be noted that in FIG. 14A, a plating layer is shown cut midway in the opening, and in FIG. 10B the plating layer is omitted and a connecting portion is exposed. Furthermore, identical symbols are attached to the same structures as in Conventional Example 1 and detailed description thereof is omitted.
With Conventional Example 5, the opening 116 of the film cover layer 115 is not formed before thermocompression bonding, but the film cover layer 115 is thermocompression bonded onto the base material, after which the opening 116 is provided by etching the film cover layer 115 and the adhesive layer 114.
In Conventional Example 5, since the opening 116 is formed after the film cover layer 115 is thermocompression bonded, no adhesive agent outflow portion 114a is produced. However, since it is necessary to perform etching on the film cover layer 115 and the adhesive layer 114, different equipment is required other than the equipment for performing patterning on the conductive layer 111, which presents problems such as maintaining equipment and increased costs, and therefore this technique is not ordinarily employed. It should be noted that Conventional Example 5 is disclosed in JP S62-113494A for example.